WASHINGTON — Of all the arguments against the Affordable Care Act that congressional Republicans are mustering in the debate over the spending bill, one hits closest to home. Congress, they say, is exempt from the very law that applies to everyone else.

The truth: Members of Congress are treated differently under Obamacare, but they're not exempt. In fact, by forcing them to purchase health insurance through publicly run exchanges, they're impacted more by that key provision than similar employees in private sector — or even in government.

But members of Congress will also be able to purchase their insurance under terms that are more favorable than other employees — in government or in business — who have access to employer-provided health care.

Here's the history: During the 2010 debate over the Affordable Care Act, Sen. Chuck Grassley, R-Iowa, proposed an amendment requiring members of Congress and their staffs to purchase health insurance though state exchanges. Democrats, viewing the amendment as a political stunt, co-opted the idea as their own and inserted it into the bill.

But the provision was silent about who would pay for that insurance, or how those payments would be treated. The exchanges were intended for uninsured people who couldn't get health insurance through their employer or qualify for Medicaid. Those who had access to health benefits meeting minimum coverage levels could still purchase insurance on the exchanges — but without a subsidy and using after-tax income.

Holding members of Congress and their staffs to that standard would have the effect of stripping them of the employer-paid health coverage they currently get, which is the same as any other federal employee. So the Office of Personnel Management issued a proposed rule in August making clear that the government would continue to pay the employer contribution for congressional health benefits at the same rate as if members were still on the federal plan.

Grassley now says that was his intent all along. "My goal, regardless of how the amendment was worded … was that we need to go into the exchange so that we would have to go through the same red tape as every other citizen," he told Roll Call Thursday. But because of what Grassley called a "drafting error," the amendment left out language that would have explicitly given lawmakers the same before-tax employer contribution as any other federal employee gets.

Some of Grassley's Republican colleagues have a different interpretation. Sen. David Vitter, R-La., introduced an unsuccessful amendment to the spending bill that would have required members of Congress to pay the full cost of their health care.

"We're on a timetable, a collision course, where unless we act a horrible policy and illegal Obama administration rule will go into effect," Vitter said on the Senate floor Thursday. "And the rule says for members and any staff who do go to the exchange, they do get to take a big fat taxpayer-funded subsidy with them — a subsidy that's completely unavailable to any other American at that income level going into the exchange."

To complicate matters, some congressional staffers would not be subject to the special treatment of Congress under Obamacare. The Grassley amendment applies to "all full-time and part-time employees employed by the official office of a member of Congress, whether in Washington, D.C., or outside Washington, DC."

The Office of Personnel Management said "official office" might not include staffers who work for a committee or leadership office — and left it up to each member of Congress to decide which staffers would be forced out of the federal health plan and into the exchanges.